A number of patents and publications are cited herein in order to more fully describe and disclose the invention and the state of the art to which the invention pertains. Each of these references is incorporated herein by reference in its entirety into the present disclosure, to the same extent as if each individual reference was specifically and individually indicated to be incorporated by reference.
Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise,” and variations such as “comprises” and “comprising,” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.
It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a pharmaceutical carrier” includes mixtures of two or more such carriers, and the like.
Ranges are often expressed herein as from “about” one particular value, and/or to about another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment.
This disclosure includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
Allergic Diseases
Allergic diseases, such as asthma, rhinitis, conjunctivitis and eczema, are escalating global healthcare problems which have not been contained by existing medications. These clinical conditions are initiated and triggered in genetically susceptible individuals by exposure to a diverse range of substances known as allergens. Numerous sources of allergen exist, but those associated with domestic environments are especially important as disease triggers because people are exposed to them for long periods. Amongst domestic allergens, those derived from house dust mites (HDM) are globally the most significant cause of allergic disease. These mites are found abundantly in homes, in workplaces, in entertainment venues, and in public and private transport vehicles. Chronic sensitization to HDM allergens can occur at any time of life and subsequent exacerbations triggered by repeated allergen exposure increase the probability that minor conditions such as allergic rhinitis will escalate into asthma, which is more serious. In addition, house dust mites create health problems for animals that co-habit with humans. For example, the condition of canine atopy is an inherited condition that gives rise to a miscellany of allergic conditions of the skin, nose and eyes (Sture et al., 1995). Perennial symptoms are commonly associated with sensitization and subsequent re-exposure to dust mite allergens. It is well-described with house dust mites recognised as significant triggers of perennial allergic symptoms in dogs, resulting in a need for veterinary treatment to alleviate disease symptoms. The symptoms seen in dogs largely resemble those seen in human atopic dermatitis and conjunctivitis.
The pre-eminence of house dust mite allergens as triggers of allergic conditions has resulted in a need to understand why they are allergenic. Studies into the molecular basis of allergenicity have revealed that the HDM allergen of greatest clinical significance is a cysteine peptidase. Surprisingly, this peptidase activity contributes decisively to the development of allergy to HDM allergens generally and to other by-stander allergens unrelated to HDM.
Several species of dust mite are known (e.g., Dermatophagoides pteronyssinus, Dermatophagoides farinae, Dermatophagoides siboney and Euroglyphus maynei) and each of these produce numerous allergenic proteins. The allergens from the different species can be categorized into distinct groups which show immunological cross-reactivity because they are highly identical proteins with conserved amino acid sequences. In the case of HDM, the Group 1 allergens (e.g., Der p 1, Der f 1, Eur m 1) underlie >95% of HDM allergy and are a highly conserved family of cysteine peptidases. The normal function of these cysteine peptidases in mites is as digestive enzymes which have the capability of digesting the resilient structural proteins in dried flakes of exfoliated skin which form a significant component of the HDM diet. The degree of amino acid sequence conservation in HDM Group 1 cysteine peptidase allergens (>90%) is such that they may be regarded as functionally identical and, for drug discovery purposes, a single therapeutic target. It is also now known that a clinically significant allergen from another mite of more restricted geographical distribution, Blomia tropicalis, is a related cysteine peptidase and shows immunological reactivity with the Group 1 allergens from house dust mites. This suggests that an inhibitor of Group 1 HDM allergens may be more generally applicable as inhibitors of related molecules in all species of mite that cause allergy.
The Group 1 HDM allergens are major triggers of asthma and other allergic conditions. When inhaled, their peptidase activity cleaves proteins that (i) increases the permeability of the airway epithelium allowing access for them and other, non-peptidase allergens to dendritic antigen presenting cells, and (ii) triggers signalling events that skew immunological responses to the Th2 phenotype. Both of these events initiate allergy and must be recapitulated to maintain it. Blocking these essential, top-level steps in allergic sensitization by inhibiting the cysteine peptidase activity of the Group 1 allergens could therefore provide the basis for a unique approach to the treatment and prevention of allergy.
Group 1 HDM Allergens as a Therapeutic Target
People are exposed to house dust mite (HDM) allergens for up to 23 hours each day; consequently these allergens are of major clinical significance in a range of clinical conditions that share elevated IgE as a molecular marker of disease. Population-based cross-sectional and longitudinal studies demonstrate that a positive skin test reaction for IgE antibody to HDM allergens is associated with asthma, persistent rhinitis, allergic conjunctivitis or atopic dermatitis (Arruda et al., 1991; Gelber et al., 1993; Miyamoto et al., 1968; Peat et al., 1996; Peat et al., 1991; Pollart et al., 1989; Smith et al., 1969; Sporik et al., 1990) In genetically predisposed individuals, first encounters with these allergens can trigger the onset of disease at any time and, with repeated exposures through life, minor conditions can evolve into serious disease. Thus, the probability of developing asthma is increased 10-20 fold after rhinitis has been established. Furthermore, the largest ever study of adult-onset asthma demonstrated, contrary to previous beliefs, that HDM allergy is as important to adults as children (Jaakkola et al., 2006).
Allergy risk and severity both show dose-response relationships with allergen exposure. This increases the attraction of pharmacological intervention aimed at Group 1 HDM allergens. Clinical evidence strongly supports a threshold level of exposure above which sensitization of at-risk individuals becomes probable. Furthermore, a dose-response relationship exists between concentrations of these allergens in homes (and thus human exposure) and the importance of this sensitization to asthma (Gelber et al., 1993; Peat et al., 1991; Platts-Mills et al., 1997; Platts-Mills et al., 1987; Dowse et al., 1985; Charpin et al., 1991). These observations imply that avoidance or inactivation of these key allergens (i.e., by reducing the dose of functional allergen to which an individual is exposed) is likely to decrease sensitization, causing symptoms to wane and clinical prognosis to improve. Reducing exposure to these allergens is the basis of physical allergen avoidance strategies which have been investigated as a means of controlling allergy. The benefits of physical allergen avoidance are supported by controlled trials in which people have been moved to environments (e.g., alpine sanatoria) where allergen avoidance can be managed rigorously (Dowse et al., 1985; Platts-Mills et al., 2000; Vervloet et al., 1982; Peroni et al., 1994). The effect of a strict regime of allergen avoidance is rapid in onset, with patients showing a significant decrease in markers of inflammation or medicine usage within 2 to 4 weeks (van Velzen et al., 1996; Schultze-Werninghaus, 2006; Bodini et al., 2004; Gourgoulianis et al., 2001; Piacentini et al., 1999; Piacentini et al., 1998). However, such physical avoidance measures are generally impractical and the benefits wane upon a return to everyday life.
Given the contribution of proteolytic activity to allergic sensitization, the development of a means to inhibit the peptidase activity of Group 1 allergens would provide pharmacological allergen inactivation that would mimic the effects of physical allergen avoidance. It is envisaged that the optimum means to achieve this objective would be to treat patients with such inhibitors, either topically or systemically. One advantage of this approach is that pharmacological allergen inactivation would travel with the person being treated (i.e., it would be “portable”) to achieve the benefits of continuous allergen avoidance, something which is not achievable with physical allergen avoidance measures. In addition to their use as medicines, it is likely that inhibitors of Group 1 peptidase allergens would have additional value as acaricides applied as environmental treatments. By inactivating key enzymes involved in the digestion of food by HDM, such inhibitors would deprive mites of a source of nutrition causing them to fail to thrive.
Allergens and Peptidase Activity
Two observations are relevant to an appreciation of the contribution of peptidase activity to allergic sensitization. The first is the demonstration that the proteolytic activity of a small cadre of enzymatic allergens is vital to allergic sensitization via the airways. Secondly is the ability of peptidases to drive allergic sensitization to by-stander allergens that lack proteolytic activity. When administered alone and without adjuvants, such non-enzymatic bystanders fail to evoke responses, induce tolerance or show only weak IgG-mediated reactions, even with systemic immunisation (Seymour et al., 1998; van Halteren et al., 1997; McMillan et al., 2004; McCusker et al., 2002; Hellings et al., 2001). Since the majority of allergens are non-proteolytic, the ability of individual peptidases to exert a marked influence on the development of sensitization to by-stander allergens creates an interesting therapeutic opportunity which inhibitors of Group 1 mite allergens could exploit.
Previous studies have shown that the proteolytic activity of Group 1 HDM allergens makes an essential contribution to allergy through two general mechanisms that are central to the initiation and maintenance of the allergic state. These are:                Facilitating allergen delivery across mucosal surfaces, thus gaining access to antigen presenting cells (e.g., in the lungs, dendritic cells) (Holt et al., 1990; Holt, 2002; Huh et al., 2003; Lambrecht et al., 2003a; Lambrecht et al., 2002; Lambrecht et al., 2003b; Wan et al., 2000).        Activating signal transduction pathways that favour development of allergy in the genetically predisposed (Hellings et al., 2001; Comoy et al., 1998; Stewart et al., 2003).        
HDM peptidase allergens therefore exert significant effects that are independent of IgE, but which have an essential bearing on IgE sensitization and allergic responses (King et al., 1998; Asokananthan et al., 2002). These actions serve to promote sensitization to the inciting peptidase allergen but, as described above, because the effects of the general mechanisms are essentially allergen non-specific, sensitization to non-enzymatic bystander allergens also occurs (Stewart et al., 2003; Wan et al., 1999).
Allergen Delivery
Dendritic cells are the primary antigen presenting cells of the respiratory tract (Holt et al., 1990; Holt, 2002; Huh et al., 2003; Lambrecht et al., 2003a; Lambrecht et al., 2002; Lambrecht et al., 2003b). However, for effective IgE responses to develop and be maintained, the probability of contact with antigens must be increased (Lambrecht et al., 2003b). This essential step in the detection of allergen is facilitated by the cysteine peptidase activity of Group 1 mite allergens which cleaves the transmembrane adhesion proteins of epithelial tight junctions, facilitating paracellular delivery of any allergen to dendritic cells (Wan et al., 1999; Wan et al., 2000; Winton et al., 1998).
IgE-Independent Cell Activation
Peptidase allergens are thought to contribute to innate immunity and activate a variety of cells by numerous IgE-independent mechanisms. Signalling pathways activated by cleavage of tethered ligand receptors on epithelial cells is one such mechanism contributing to the chronic release of GM-CSF and IL-6. These cytokines are present in increased amounts in the airways in allergic asthma and rhinitis (Broide et al., 1992; Fahy et al., 1995; Muraguchi et al., 1988; Vercelli., 1989). They promote a Th2 allergic bias via several actions. For example, IL-6 is essential to B cell maturation and in the IL-4-dependent synthesis of IgE (Muraguchi et al., 1988; Vercelli., 1989). GM-CSF generates signals that cause dendritic cells to migrate from the airway epithelium to present captured antigens at regional lymph nodes (Stick et al., 2003). Proteolytic activity that cleaves tethered ligand receptors is thus associated with a chain of events central to both the initiation of allergic sensitization and its maintenance. Peptidase allergens activate mast cells by IgE-independent mechanisms and it follows, therefore, that a contribution to the acute bronchoconstriction resulting from allergen challenge must be due to this peptidase-dependent activation. This suggests that inhibitors of Group 1 peptidase allergens should attenuate acute allergic bronchoconstriction. Other IgE-independent mechanisms involve a cleavage of cytokine and IgE receptors that are associated with an augmentation of allergy (Ghaemmaghami et al., 2002), cleavage of antipeptidase defences (which may already be defective in allergy) and cleavage of other protective factors such as surfactant proteins (Deb et al., 2007).
Demonstrations of Proteolytic Allergen Contributions to Allergy
The potential importance of peptidase allergens as a target in allergy is demonstrated by the ease and directness with which they evoke IgE sensitization and by studies with generic inhibitors of cysteine peptidases in experimental animals.
Strong allergen-specific IgE sensitization can be achieved by non-invasive exposure of mice to Der p 1 of high specific proteolytic activity in the absence of adjuvants (Zhang et al., 2009). In Brown Norway rats, development of Der p 1-specific IgE and allergic responsiveness also occurs without the need for additional adjuvants. In contrast, the difficulties in raising high titre antibodies to recombinant Der p 1 that lacks high enzyme activity (and which therefore behaves like a by-stander allergen) are well known. The proteolytic nature of Der p 1 also augments the sensitization to non-peptidase bystander allergens from HDM and other sources (Gough et al., 2001).
The promotion of allergen delivery by peptidase allergens may be augmented by their inactivation of antipeptidase defences (Kalsheker et al., 1996). Of related significance is that the loss of functional polymorphisms in endogenous enzyme inhibitors (e.g., chromosome 5q32 LETK1, chromosome 7 PAI-I, chromosome 11 C1 esterase inhibitor, chromosome 14 serpin cluster, chromosome 18q21) predisposes the subject to allergic disease. This recent evidence supplements functional associations between allergy and protease inhibitor deficiency that have accrued over the past 25 years (Rudolph et al., 1978; Hyde et al., 1979; Eden et al., 2003; Sigsgaard et al., 2000).